This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A105632 #19 Jun 12 2023 09:20:42
%S A105632 1,1,1,2,1,1,4,3,1,1,9,7,4,1,1,21,19,10,5,1,1,51,51,31,13,6,1,1,127,
%T A105632 141,91,45,16,7,1,1,323,393,276,141,61,19,8,1,1,835,1107,834,461,201,
%U A105632 79,22,9,1,1,2188,3139,2535,1485,701,271,99,25,10,1,1,5798,8953,7711,4803,2381,1001,351,121,28,11,1,1
%N A105632 Triangle, read by rows, where the g.f. A(x,y) satisfies the equation: A(x,y) = 1/(1-x*y) + x*A(x,y) + x^2*A(x,y)^2.
%C A105632 Column 0 is A001006 (Motzkin numbers). Column 1 is A002426 (Central trinomial coefficients). Row sums form A105633 (also equal to A057580?).
%C A105632 T(n,k) is the number of UUDU-avoiding Dyck paths of semilength n+1 with k UDUs, where U = (1,1) is an upstep and D = (1,-1) is a downstep. For example, T(3,1) = 3 counts UDUUUDDD, UDUUDDUD, UUDDUDUD. - _David Callan_, Nov 25 2021
%F A105632 G.f. for column k (k>0): Sum_{j=0..k-1} C(k-1, j)*A000108(j)*x^(2*j)/(1-2*x-3*x^2)^(j+1/2), where A000108(j) = binomial(2*j, j)/(j+1) is the j-th Catalan number.
%F A105632 G.f.: A(x, y) = (1-x - sqrt((1-x)^2 - 4*x^2/(1-x*y)))/(2*x^2).
%e A105632 Triangle begins:
%e A105632 1;
%e A105632 1, 1;
%e A105632 2, 1, 1;
%e A105632 4, 3, 1, 1;
%e A105632 9, 7, 4, 1, 1;
%e A105632 21, 19, 10, 5, 1, 1;
%e A105632 51, 51, 31, 13, 6, 1, 1;
%e A105632 127, 141, 91, 45, 16, 7, 1, 1;
%e A105632 323, 393, 276, 141, 61, 19, 8, 1, 1;
%e A105632 835, 1107, 834, 461, 201, 79, 22, 9, 1, 1; ...
%e A105632 Let G = (1-2*x-3*x^2), then the column g.f.s are:
%e A105632 k=1: 1/sqrt(G)
%e A105632 k=2: (G + (1)*1*x^2)/sqrt(G^3)
%e A105632 k=3: (G^2 + (1)*2*x^2*G + (2)*1*x^4)/sqrt(G^5)
%e A105632 k=4: (G^3 + (1)*3*x^2*G^2 + (2)*3*x^4*G + (5)*1*x^6)/sqrt(G^7)
%e A105632 k=5: (G^4 + (1)*4*x^2*G^3 + (2)*6*x^4*G^2 + (5)*4*x^6*G + (14)*1*x^8)/sqrt(G^9)
%e A105632 and involve Catalan numbers and binomial coefficients.
%e A105632 MATRIX INVERSE.
%e A105632 The matrix inverse starts
%e A105632 1;
%e A105632 -1, 1;
%e A105632 -1, -1, 1;
%e A105632 0, -2, -1, 1;
%e A105632 2, -1, -3, -1, 1;
%e A105632 6, 2, -2, -4, -1, 1;
%e A105632 13, 10, 2, -3, -5, -1, 1;
%e A105632 18, 32, 14, 2, -4, -6, -1, 1;
%e A105632 -12, 76, 56, 18, 2, -5, -7, -1, 1;
%e A105632 -206, 108, 162, 86, 22, 2, -6, -8, -1, 1;
%e A105632 - _R. J. Mathar_, Apr 08 2013
%p A105632 A105632 := proc(n,k)
%p A105632 (1-x-sqrt((1-x)^2-4*x^2/(1-x*y)))/2/x^2 ;
%p A105632 coeftayl(%,x=0,n) ;
%p A105632 coeftayl(%,y=0,k) ;
%p A105632 end proc: # _R. J. Mathar_, Apr 08 2013
%t A105632 T[n_, k_] := SeriesCoefficient[(1 - x - Sqrt[(1 - x)^2 - 4*x^2/(1 - x*y)])/(2*x^2), {x, 0, n}] // SeriesCoefficient[#, {y, 0, k}]&;
%t A105632 Table[T[n, k], {n, 0, 11}, {k, 0, n}] // Flatten (* _Jean-François Alcover_, Jun 10 2023 *)
%o A105632 (PARI) {T(n,k)=local(A=1+x+x*y+x*O(x^n)+y*O(y^k)); for(i=1,n,A=1/(1-x*y)+x*A+x^2*A^2);polcoeff(polcoeff(A,n,x),k,y)}
%o A105632 (PARI) {T(n,k)=local(X=x+x*O(x^n),Y=y+y*O(y^k));polcoeff(polcoeff( 2/(1-X+sqrt((1-X)^2-4*X^2/(1-X*Y)))/(1-X*Y),n,x),k,y)}
%Y A105632 Cf. A105633 (row sums), A001006 (column 0), A002426 (column 1).
%K A105632 nonn,tabl
%O A105632 0,4
%A A105632 _Paul D. Hanna_, Apr 17 2005